A tubing string which is normally suspended from the wellhead may be several thousand feet in length and it is desirable to place the tubing under some tension, particularly when the tubing string might be subjected, for example, to relatively large variations in temperature, vibrations, a pumping action, or the cycling of certain fluids. The desired amount of tensioning for the tubing string may be estimated depending on various factors such as the thickness of the pipe string, the type of pipe string material, the length of the string, and the expected temperature changes.
For stretching or lifting a tubing string, the lower end of the tubing string is anchored such as by a suitable tension packer which may be easily run and set at the desired depth by a suitable tool such as by rotation of the tubing or tool to actuate the packer. A mandrel is normally threaded within a hanger supporting the upper end of the tubing string. The mandrel is lifted by a suitable hoisting mechanism along with the hanger and tubing string from a position below the wellhead with the amount of tension being calculated to indicate the distance that the hanger and tubing string is to be lifted for the desired tensioning of the tubing string.
Heretofore, separate members had been utilized for seating the tubing string in the wellhead assembly after the tensioning of the tubing string. As an example, reference is made to U.S. Pat. No. 3,011,552 dated Dec. 5, 1961 in which an apparatus for setting pipes in tension is illustrated. The lower end of the tubing string is anchored and the tubing string is "stretched" or pulled upwardly for the desired amount of tension. Then a sleeve is positioned around the tubing string joint and seated to maintain the tensioning. It is apparent that the positioning and seating of the separate sleeve by suitable threads is time consuming and requires close fitting elements.